LiDFOB - Chemical & Biomolecular Engineering - North Carolina ...

Article 

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Solvate Structures and Computational/Spectroscopic 

Characterization of Lithium Difluoro(oxalato)borate (LiDFOB) 

Electrolytes 

Sang-Don Han, † Joshua L. Allen, † Erlendur Jońsson, ‡ Patrik Johansson, ‡ Dennis W. McOwen, † 

Paul D. Boyle, § and Wesley A. Henderson* ,† 

† Ionic Liquids & Electrolytes for Energy Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering, 

North Carolina State University, Raleigh, North Carolina 27695, United States 

‡ Department of Applied Physics, Chalmers University of Technology, SE-412 96, Gothenburg, Sweden 

§ X-ray Structural Facility, Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States 

*S Supporting Information 

ABSTRACT: Lithium difluoro(oxalato)borate (LiDFOB) is a relatively new salt designed for 

battery electrolyte usage. Limited information is currently available, however, regarding the 

ionic interactions of this salt (i.e., solvate formation) when it is dissolved in aprotic solvents. 

Vibrational spectroscopy is a particularly useful tool for identifying these interactions, but only 

if the vibrational bands can be correctly linked to specific forms of anion coordination. Single 

crystal structures of LiDFOB solvates have therefore been used to both explore the 

DFOB − ...Li + cation coordination interactions and serve as unambiguous models for the 

assignment of the Raman vibrational bands. The solvate crystal structures determined include 

(monoglyme) 2 :LiDFOB, (1,2-diethoxyethane) 3/2 :LiDFOB, (acetonitrile) 3 :LiDFOB, (acetonitrile) 

1 :LiDFOB, (dimethyl carbonate) 3/2 :LiDFOB, (succinonitrile) 1 :LiDFOB, (adiponitrile) 

1 :LiDFOB, (PMDETA) 1 :LiDFOB, (CRYPT-222) 2/3 :LiDFOB, and (propylene carbonate) 

1 :LiDFOB. DFT calculations have been incorporated to provide additional insight into the 

origin (i.e., vibrational modes) of the Raman vibrational bands to aid in the interpretation of 

the experimental analysis. 

1. INTRODUCTION 

The difluoro(oxalato)borate (DFOB − ) anion, also known as 

oxalyldifluoroborate (ODFB − ), was first reported in a patent 

application filed in 1999 by Metallgesellschaft AG 1 and in a 

publication in 2006 by the U.S. Army Research Laboratory 

(ARL). 2 The lithium salt with this anion (LiDFOB) has 

garnered significant interest in recent years for Li-ion battery 

electrolytes. 3−39 Little is known at present, however, about the 

molecular-level ionic interactions present once this salt is 

dissolved in the aprotic solvents used for electrolytes. Such 

knowledge is necessary to understand the origin of electrolyte 

properties as solvent−salt mixtures consist of a diverse range of 

solvate species. 40,41 

The solvates present in electrolytes may be broadly classified 

as solvent-separated ion pair (SSIP), contact ion pair (CIP) or 

aggregate (AGG) solvates if the anions are coordinated to zero, 

one or more than one Li + cation, respectively. Vibrational 

spectroscopy (Raman, FTIR) is the predominant means for 

determine the extent of ionic association in electrolytes by 

examining the variation in anion vibrational band positions. 

When an anion forms one or more coordination bonds to Li + 

cations, the anion electron density distribution (and thus bond 

lengths and angles) changes, leading to vibrational band 

variation. For Raman spectroscopy, the variability of the solvate 

species present in solution, however, typically results in broad 

overlapping anion Raman bands which can be difficult to 

deconvolute correctly and thus properly interpret to distinguish 

the specific forms of anion...Li + cation coordination. Computational 

methods may be utilized to assign the band positions for 

varying forms of coordination. 42−46 Unfortunately, such studies, 

while extremely useful for understanding the anion vibrational 

bands, do not always provide a definitive validation for the 

assignments. It has previously been demonstrated, however, 

that crystalline solvates with known structures can be utilized as 

models for the spectroscopic characterization of different forms 

of ClO 4 − ...Li + cation coordination. 47−52 These analyses are of 

particular relevance as they indicate that many of the vibrational 

assignments previously published for the coordination found in 

LiClO 4 -based electrolytes are either incorrect or oversimplifications 

of the true ionic association state present in the 

electrolytes. 

Received: September 13, 2012 

Revised: February 1, 2013 

Published: February 5, 2013 

© 2013 American Chemical Society 5521 dx.doi.org/10.1021/jp309102c | J. Phys. Chem. C 2013, 117, 5521−5531

The Journal of Physical Chemistry C 

Although useful for discussions, the general classification of 

SSIP, CIP, and AGG solvates fails to fully account for the 

coordination complexity that may exist. This classification must 

therefore be further differentiated to include specific forms of 

anion coordination, examples of which are shown in Chart 1 for 

Article 

Chart 2. Structures and Acronyms of the Solvents Studied 

Chart 1. DFOB ‐ ...Li + Cation Coordination: (a) SSIP, (b) 

CIP-I, (c) CIP-II, (d) AGG-Ia, (e) AGG-Ib, (f) AGG-Ic, (g) 

AGG-Id, (h) AGG-IIa, (i) AGG-IIb, (j) AGG-IIIa, and (k) 

AGG-IIIb a 

a Other forms of coordination may also occur. Li + cations are colored 

black. 

the DFOB − anion. Each of these different forms of 

coordination results in a different vibrational spectral fingerprint. 

To explore this in depth, the crystal structures for ten 

new crystalline solvate structures with LiDFOB are reported 

here. The Raman spectroscopic characterization of these 

solvates is also provided, as well as the data for the salt 

LiDFOB and other LiDFOB solvates for which a structure is 

known or can be reasonably presumed. In addition, density 

functional theory (DFT) calculations for the uncoordinated 

(i.e., SSIP) DFOB − anion and the CIP-II (Chart 1) form of 

DFOB − ...Li + cation coordination are presented to elucidate the 

vibrational modes which contribute to the vibrational bands 

and their correlation with the spectra for the crystalline solvates. 

2. EXPERIMENTAL SECTION 

2.1. Materials. LiDFOB was synthesized by the direct 

reaction of excess boron trifluoride diethyl etherate (BF 3 -ether) 

with lithium oxalate (oxalic acid dilithium salt), both used asreceived 

from Sigma-Aldrich. The resulting salt was extracted/ 

recrystallized several times from dimethyl carbonate (DMC) as 

a (DMC) 3/2 :LiDFOB crystalline solvate. This solvate was 

subsequently vacuum-dried at 105 °C for 48 h, yielding the 

high purity anhydrous LiDFOB salt free of solvent. 11 

The solvents used in the present study and their acronyms 

are noted in Chart 2. The ethylene glycol dimethyl ether or 1,2- 

dimethoxyethane (monoglyme or G1, anhydrous, 99.5%), 

diethylene glycol dimethyl ether or 2-methoxyethyl ether 

(diglyme or G2, anhydrous, 99.5%), ethylene glycol diethyl 

ether or 1,2-diethoxyethane (Et-G1, 98%), N,N,N′,N″,N″- 

pentamethyldiethylenetriamine (PMDETA, 99%), 

1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA, 

97%), 1,4,7,10-tetraoxacyclododecane (12-crown-4 or 12C4, 

98%), 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]- 

hexacosane (CRYPT-222, 98%), acetonitrile (AN, extra dry, 

99.9%), succinonitrile (SN, 99%), adiponitrile (ADN, 99%), 

tetramethylene sulfone or sulfolane (TMS, 99%), propylene 

carbonate (PC, anhydrous, 99.7%) and dimethyl carbonate 

(DMC, anhydrous, ≥ 99%) were purchased from either Fisher 

Scientific or Sigma-Aldrich and used as-received. The water 

content of the solvents was verified to be negligible (


corrected using a multiscan averaging of symmetry equivalent 

data using SADABS. 54 The structures were solved by direct 

methods using the SIR92 54 or XS 55 programs. All nonhydrogen 

atoms were obtained from the initial solution. The 

hydrogen atoms were introduced at idealized positions and 

were allowed to ride on the parent atom. The structural models 

were fit to the data using full matrix least-squares based on F 2 . 

The calculated structure factors included corrections for 

anomalous dispersion from the usual tabulation. The structures 

were refined using the XL program from SHELXTL. 55 

Additional details are available in the Supporting Information 

(SI). Structures were drawn using Ortep-3 or Mercury 3.0 

software. Crystallographic information data files (CIFs) for the 

solvates are available in the SI and free of charge from the 

Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. 

uk/data_request/cif: CCDC 896911 (ADN) 1 :LiDFOB, 

896912 (AN) 1 :LiDFOB, 896913 (AN) 3 :LiDFOB, 896914 

(CRYPT-222) 2/3 :LiDFOB, 896915 (DMC) 3/2 :LiDFOB, 

896916 (Et-G1) 3/2 :LiDFOB, 896917 (G1) 2 :LiDFOB, 896918 

(PC) 1 :LiDFOB, 896919 (PMDETA) 1 :LiDFOB and 896920 

(SN) 1 :LiDFOB. Mercury freeware software to view the content 

of these files is available: www.ccdc.cam.ac.uk/free_services/ 

mercury/downloads/. 

2.4. Elemental Analysis. The analysis of crystals for the 

(12C4) 2 :LiDFOB and (HMTETA) 1 :LiDFOB solvates was 

performed by Atlantic Microlab, Inc.C 18 H 32 O 12 F 2 BLi for 

(12C4) 2 :LiDFOB, calcd: C 43.57, F 7.66, H 6.50; found: C 

43.50, F 7.53, H 6.36 and C 14 H 34 O 4 F 2 BLiN 4 for (HMTE- 

TA) 1 :LiDFOB, calcd: C 44.46, N 14.81, F 10.05 H 9.06; found: 

C 44.45, N 14.78, F 9.86, H 8.39confirming these as the 

correct compositions. A similar analysis was not performed on 

crystals for the (G2) 2 :LiDFOB solvate as this solvate melts 

below ambient temperature (SI). 

2.5. Raman Spectroscopy. Raman vibrational spectra 

were collected with a Horiba-Jobin Yvon LabRAM HR VIS 

high resolution confocal Raman microscope using a 632 nm 

He−Ne laser as the exciting source and a Linkam stage for 

temperature control with a long distance 50× objective. An 

elemental Si reference (520.7 cm −1 ) was used for spectral axis 

calibration. The zero position of the laser line and the Si line 

position were aligned prior to the data collection for each 

sample. The samples were hermetically-sealed in the Linkam 

stage in the glovebox before transferring the stage to the 

spectrometer. Spectra were typically collected using a 20 s 

measurement time with 10 accumulations. The samples were 

cooled/heated at 5 °C min −1 and the spectra were collected 

from −100 to 60 °C at intervals of 20 °C. Raman spectra were 

processed using LabSpec software. 

2.6. Computational Details. The DFOB − anion and its 

various ion pairs with a Li + cation were constructed manually 

and geometry optimized using the 6-311+G* basis set 

employing two different DFT functionals: B3LYP 56−58 and 

M06−2X 59 the former to support backward compatibility 

(significant previous work has been done using only the B3LYP 

functional) and the latter to make use of the recent 

development of better functionals (M06−2X). Utilizing both 

serves as an internal calibration/verification of the results. 

Subsequently, the structures were all verified as energy minima 

by computing their Hessians and the Raman spectral data 

(frequencies, activities) were obtained from the partial third 

derivatives of the energy. In addition to the above calculations, 

all done for ionic species in vacuum, a solvent effect was added 

(with the structures reoptimized) using a solvent self-consistent 

Article 

reaction field (SCRF-SMD) methodology 60 with AN as the 

parametrized solvent. This was done to in some way account 

for the reduced ion−ion interactions in the crystalline and/or 

liquid environment, and thus to mimic the experimental 

analysis to some extent. As the DFOB − anion is able to adopt a 

multitude of different forms of Li + cation coordination, the 

same strategy as noted above was also employed to scrutinize 

various ion pairs for numerous DFOB − ...Li + cation combinations. 

All calculations were made using Gaussian 09. 61 To assist 

the comparison with the experimental data, artificial Raman 

spectra were created by convolution of the spectral data using a 

Lorentzian band shape and a fwhm of 5 cm −1 . 

3. RESULTS AND DISCUSSION 

Although vibrational spectroscopy is a powerful tool for 

delineating the anion interactions with Li + cations, limitations 

exist for transcribing such information into a robust understanding 

of the ionic association interactions in solution. This is 

evident from a scrutiny of the solvate crystal structures. Figure 

1 shows examples of two different forms of anion CIP 

Figure 1. DFOB − ...Li + cation coordination in the crystalline solvates: 

(a) (G1) 2 :LiDFOB and (b) (Et-G1) 3/2 :LiDFOB (Li-purple, O-red, B- 

tan, F-light green). 

coordination in the (G1) 2 :LiDFOB and (Et-G1) 3/2 :LiDFOB 

crystalline solvates. Vibrational spectroscopy only indirectly 

provides information about the Li + cation interactions. Thus, 

for the (Et-G1) 3/2 :LiDFOB crystalline solvate, the spectroscopic 

data would indicate a mixture of CIP-I and CIP-II 

solvates. But, from the perspective of the Li + cation 

coordination, the structure instead consists of SSIP and AGG 

solvates. Similarly, the spectroscopic data for the (CRYPT- 

222) 2/3 :LiDFOB solvate correspond to CIP-I/CIP-II/AGG-Id 

coordination (three symmetrically independent anions), whereas 

the Li + cation coordination is SSIP/SSIP/AGG. Molecular 

dynamics (MD) simulations of electrolyte mixtures point to a 

further complexity. In the liquid mixtures, there is a distribution 

of fully solvated Li + cations and uncoordinated anions, ion pairs 

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dx.doi.org/10.1021/jp309102c | J. Phys. Chem. C 2013, 117, 5521−5531


Article 

Figure 2. DFOB − ...Li + cation coordination in crystalline LiDFOB and the crystalline solvates studied: (a) AGG-IIIb LiDFOB, 11 (b) AGG-Ic 

(H 2 O) 1 :LiDFOB, 11 (c) CIP-I (TMS) 2 :LiDFOB, 16 (d) CIP-II (G1) 2 :LiDFOB, (e) CIP-I/CIP-II (Et-G1) 3/2 :LiDFOB, (f) AGG-Ia 

(DMC) 3/2 :LiDFOB, (g) AGG-Ib (SN) 1 :LiDFOB, (h) AGG-Ia (AN) 3 :LiDFOB, and (i) AGG-IIa (AN) 1 :LiDFOB. 

and aggregate clusters of ions which may be present. 40,41 These 

solvated aggregate clusters may contain one or more anions 

coordination to one Li + cation (i.e., CIP coordination), but the 

cluster itself may actually be quite large with many ions (i.e., the 

Li + cation may be bonded to additional anions). 40 Thus, the 

spectroscopic data may again not be indicative of the true state 

of ionic association. Caution should therefore be exercised in 

directly translating the information obtained from a spectral 

analysis of electrolytes into an interpretation of solvate 

distribution as doing so may be highly misleading. Considerable 

insight from such an analysis may be obtained, however, when 

the spectroscopic data are used in tandem with other 

methods. 40,41 

3.1. Solvate Structures and Li + Cation Coordination. 

Figures 2 and 3 show schematic illustrations of the DFOB − ...Li + 

cation coordination in the crystalline solvates. The structures 

for pure LiDFOB, (H 2 O) 1 :LiDFOB and (TMS) 2 :LiDFOB have 

been previously reported. 11,16 The remaining structures were 

determined as part of the present study (SI). The DFOB − 

anion is able to adopt a multitude of different forms of Li + 

cation coordination. All of the anions in the solvate structures 

have at least one of the carbonyl oxygen atoms coordinated to a 

Li + cation. The two carbonyl oxygen atoms may coordinate up 

5524 

to three Li + cations using the four electron lone-pairs available. 

Bidentate coordination of a Li + cation by both carbonyl oxygen 

atoms is found in seven of the structures: LiDFOB, 

(G1) 2 :LiDFOB, (Et-G1) 3/2 :LiDFOB, (ADN) 1 :LiDFOB, 

(SN) 1 :LiDFOB, (AN) 1 :LiDFOB and (CRYPT- 

222) 2/3 :LiDFOB. This suggests that this is a common feature 

for the anion...Li + cation interactions in the solid-state 

structures. Other anions such as bis(oxalatoborate) 

(BOB − ), 44 dicyanotriazolate (DCTA − or TADC − ) 45 and 

bis(trifluoromethanesulfonyl)imide (TFSI − ) 41 may also have 

a tendency to form bidentate coordination, but this appears to 

diverge from the behavior of anions such as ClO − 4 ,BF − 4 , and 

PF − 6 , which instead typically coordinate Li + cation in solid-state 

solvate structures and in solution (the latter from MD 

simulations) via monodentate coordination. 50,62 Only four of 

the structures contain Li + cations coordinated by fluorine 

atoms: LiDFOB, (ADN) 1 :LiDFOB, (CRYPT-222) 2/3 :LiDFOB 

and (PC) 1 :LiDFOB. In all of these solvates, the fluorine atom 

coordination serves as a necessary bridge to weld the structure 

together. This suggests that while fluorine atom coordination to 

Li + cations may occur, it is much less prevalent than the 

carbonyl oxygen atom coordination. 



Article 

Figure 3. DFOB − ...Li + cation coordination in the crystalline solvates studied: (a) AGG-IIb (ADN) 1 :LiDFOB, (b) AGG-Ia (PMDETA) 1 :LiDFOB, 

(c) CIP-I/CIP-II/AGG-Id (CRYPT-222) 2/3 :LiDFOB, and (d) AGG-IIIa (PC) 1 :LiDFOB. 

3.2. Raman Band Assignments. The experimental Raman 

spectrum for the anhydrous LiDFOB crystalline salt is shown in 

Figure 4. To aid in the assignment of the peaks, the DFT 

computed vibrational modes for the uncoordinated DFOB − 

anion are shown in Table 1. Note that differences are to be 

expected between the spectra for the anions in the highly 

aggregated salt and the uncoordinated anions. The vibrational 

modes may be visualized (through animation) using the 

Gaussian 09 output files (.xyz format) and instructions 

provided in the SI. 

The two strong peaks found above 1700 cm −1 in Figure 4 

correspond to the CO stretching vibrationsantisymmetric 

and symmetric, respectively (Table 1 and Chart 3 which note 

that the DFT calculations predict too high a frequency for these 

by ∼100 cm −1 ). Judging from the structural data, these bands 

should be excellent probes for the DFOB − ...Li + cation 

interactions as these occur principally via the anion carbonyl 

groups. This was found, however, not to be the case. A 

summary of the symmetric CO stretching band positions for 

the crystalline solvates is given in the SI. In contrast to 

expectations, this analysis suggests that the band positions for 

these DFOB − anion carbonyl vibrational modes do not enable 

the facile discrimination of the different forms of anion 

coordination. 

Figure 4. Raman spectrum of LiDFOB (at 20 °C). 

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Table 1. Calculated (Uncoordinated DFOB − from M06-2X) Vibration Mode Frequencies (cm −1 ), Raman Activities (Å 4 amu −1 ) 

and IR Intensities (km mol −1 ), and Tentative Assignments (No Scaling) a 

vacuum 

SMD 

freq expt freq I Raman I IR freq I Raman I IR tentative assignments 

81 1.61 2.10 82 2.55 2.53 B centered torsion 

114 0.72 0 110 1.09 0 torsion of OC−CO + ring + F−B−F 

325 331 0.28 ∼0 328 0.38 ∼0 ring twisting + torsion of F−B−F 

360 334 2.22 0.09 338 3.46 0.63 symm bend OC−O in-plane + F−B−F bend out-of-phase 

390 363 1.60 11.49 365 2.55 25.11 symm bend OC−O in-plane + F−B−F bend in-phase 

373 0.05 7.34 371 0.06 15.52 ∂(C−C) + π(F−B) 

468 0.12 19.63 467 0.42 33.07 π symm C−C 

522 0.30 25.05 517 0.44 40.49 ∂(OC−O) × 2+ρ rock B−F 

615 605 2.24 14.92 608 3.15 19.20 ∂(O−C−C) 

635 621 2.33 0.68 619 3.85 1.00 B−F scissoring + O−B symm bend 

680 690 0.44 2.78 695 0.96 2.95 ring rotation 

723 728 9.55 7.41 739 15.53 9.95 ring breathing 

840 856 0.53 ∼0 854 1.25 0.01 bend C−C (out of plane) + displacements of O 

942 957 2.82 0.49 964 5.33 2.10 symm B−O stretch + C−C stretch + B−F stetch 

987 0.04 240.71 989 0.14 358.15 antisymm B−O stretch 

1119 0.30 785.88 1078 0.63 1288.27 symm B−F stretch 

1177 0.29 382.10 1113 0.47 534.47 antisymm B−F stretch 

1290 0.01 197.31 1278 0.18 264.56 in-plane C−C bending + antisymm C−O stretch 

1405 1397 6.39 478.92 1386 7.61 677.46 symm C−O stretch + C−C stretch 

1769 1870 13.75 172.05 1865 25.17 223.30 antisymm CO stretch 

1800 1906 19.19 713.11 1900 45.02 912.41 symm CO stretch 

a The term freq expt refers to the LiDFOB salt (Figure 4). 

Chart 3. Principal Bonds/Angles Contributing to the 

DFOB − Anion Vibrational Modes as Determined from DFT 

Calculations (See SI) 

The peak observed at 1405 cm −1 (Figure 4) is attributed to 

the combination band of the C−C and symmetric C−O 

stretching modes (Table 1 and Chart 3). The peak near 942 

cm −1 (Figure 4) corresponds to a combination of symmetric 

B−O, C−C, and B−F stretching modes (Table 1 and Chart 3). 

The very strong peak near 723 cm −1 (Figure 4), predicted by 

the computations to be the third strongest peak in the Raman 

spectra, is due to a ring-breathing mode (Table 1 and Chart 3). 

The more complex modes corresponding to the two or more 

peaks observed at ∼600−620 cm −1 (Figure 4) are most likely a 

combination of different O−C−C bending modes, but 

combined also with B−F scissoring from the other half of the 

DFOB − anion (Table 1 and Chart 3). Thus, this region is more 

difficult to use as a probe for specific DFOB − ...Li + cation 

interactions. Yet, this should, at the same time, be the region 

with bands which undergo a sizable shift in position due to any 

Li + ...anion fluorine atom coordination. It is important to also 

note that boron isotopes have a natural abundance of 19.9% 10 B 

and 80.1% 11 B. Thus, vibrational modes which involve the 

displacement of the boron nucleus will give two different bands. 

This adds a further level of complexity to the band analysis for 

the DFOB − anion. 

3.3. Raman Spectroscopic Analysis of Crystalline 

Solvates. The majority of the solvates with known crystal 

structures were characterized by Raman spectroscopy to 

determine the anion band position variation with varying 

coordination and temperature. The AGG-Ic (H 2 O) 1 :LiDFOB 

solvate, however, was not analyzed as this solvate contains 

significant hydrogen bonding between the water and anions 

(which is expected to perturb the vibrational bands), nor was 

the CIP-I/CIP-II/AGG-Id (CRYPT-222) 2/3 :LiDFOB solvate 

analyzed as the crystals melt below ambient temperature and it 

was not possible to prepare the correct liquid composition (i.e., 

dissolve the appropriate amount of LiDFOB salt directly in the 

solvent). Unfortunately, the anion bands near 945 and 1405 

cm −1 overlap significantly with the peaks from many common 

solvents. The bands in the 600−650, 705−725, and 1780−1820 

cm −1 region of the spectra, however, do not and these bands 

5526 



Article 

Figure 5. Raman spectra of the DFOB − anion vibrational band variation for the (speculative) SSIP crystalline solvates: (12C4) 2 :LiDFOB, 

(G2) 2 :LiDFOB, and (HMTETA) 1 :LiDFOB (bold spectra indicate that the solvate has melted at this temperature). 

were therefore analyzed in depth. Representative data are 

shown in Figures 5 and 6. The remaining sets of data are 

provided in the SI. A summary of the 705−725 cm −1 results for 

the DFOB − anion Raman band positions as a function of 

temperature for the crystalline LiDFOB solvates is shown in 

Figure 7. 

The data in Figure 5 are for speculative SSIP solvates as no 

crystal structures are yet known for a SSIP solvate with 

LiDFOB. Attempts were made to determine the crystal 

structures of the (12C4) 2 :LiDFOB, (G2) 2 :LiDFOB, and 

(HMTETA) 1 :LiDFOB solvates, but these efforts were 

unsuccessful due to the very energetic solid−solid phase 

transitions which occur for these solvates at low temperature 

(SI) and the presence of significant disorder within the solvate 

structures in the higher temperature phases. There are no 

reported crystal structures yet known for solvates with 

HMTETA and LiX salts, but (12C4) 2 :LiX 63−67 and 

(G2) 2 :LiX 50,68−71 solvates are well-knownall of which have 

uncoordinated anions (i.e., SSIP solvates). The band positioned 

near 710 cm −1 for all three speculative SSIP solvates remains 

5527 

relatively fixed over the entire temperature range (Figures 5 and 

7). In contrast, multiple bands are present near 620 cm −1 which 

differ for the three solvates, as do the bands near 1760 and 1800 

cm −1 (Figure 5). 

The data in Figure 6 correspond to the CIP solvates with 

known crystal structures. There is rather poor conformity in the 

band positions for all three regions of the spectra for these 

solvates. The symmetric CO band near 1800 cm −1 (1906 

cm −1 from the calculations) also includes the C−O bonds 

(Chart 3). A comparison of the bond lengths/angles for the 

CIP-I anions (SI) suggests that these have a very similar 

geometry. If the bands near 1760 and 1796 cm −1 in the 

spectrum for the (Et-G1) 3/2 :LiDFOB solvate correspond to the 

CIP-I anion (rather than the CIP-II anion), then there is very 

good agreement with the same bands in the (TMS) 2 :LiDFOB 

spectrum (Figure 6), but there is then poor agreement between 

the solvates for the bands near 620 cm −1 and 710 cm −1 (Figure 

6). 

The summary of the data for the ring breathing vibrational 

mode near 710 cm −1 shown in Figure 7 suggests that it will be 



Article 

Figure 6. Raman spectra of the DFOB − anion vibrational band variation for the CIP crystalline solvates: (TMS) 2 :LiDFOB, (G1) 2 :LiDFOB and (Et- 

G1) 3/2 :LiDFOB (bold spectra indicate that the solvate has melted at this temperature). 

quite difficult to accurately deconvolute the complex spectra for 

the DFOB − anion in the liquid phase to identify specific anion 

interactions. This is due to both the numerous forms of 

DFOB − ...Li + cation coordination which are possible and the 

variability in the band positions for any given form of anion 

coordination. This latter point is particularly true for the AGG 

solvates which have considerable variability in the anion 

geometry (SI). Given that each of the anion bands originates 

from the combination of multiple vibrations (Chart 3), it is not 

surprising that the anion geometry variation results in widely 

different spectra. In general, however, the band positions in the 

710−725 cm −1 range, especially when also considering the 

other regions of the spectra, do provide an indication of the 

relative fraction of SSIP, CIP, and AGG solvates (Figure 7). For 

example, upon melting, notable changes in the spectra occur for 

the (G1) 2 :LiDFOB and (Et-G1) 3/2 :LiDFOB solvates (Figure 

6). The spectra, which bear a close resemblance to the spectra 

for the melted (G2) 2 :LiDFOB solvate, suggest that the liquid 

melts obtained from the crystalline CIP solvates likely consist of 

a significant fraction of SSIP solvates (in addition to CIP 

solvates) with a lesser amount of AGG-I and perhaps AGG-II 

solvates. This is quite interesting given that melting of solvates 

often results in increased association (desolvation) of the 

solvate species rather than increased solvation (i.e., formation 

of SSIP solvates). 40,41 

Other forms of DFOB − ...Li + cation coordination than those 

found in the crystalline solvates (i.e., Chart 1) are possible/ 

plausible in liquid electrolytes. DFT calculations identified five 

distinct CIP forms of coordination as energy minima (A−E, 

Chart 4), but only the LiDFOB-A ion pair was found to be 

strongly energetically favored for all four calculation methods 

employed (Table 2). Furthermore, as this type of bidentate 

DFOB − coordination via the carbonyl oxygen atoms is found in 

many of the crystalline solvates, as noted above, both the 

computational and experimental results suggest that there is a 

strong preference for this type of coordination. The vibrational 

modes for the LiDFOB-A ion pair are given in the SI, along 

with the Gaussian 09 output file for this ion pair. 

5528 



Figure 7. Summary of the Raman band peak position for the DFOB − 

anion ring breathing vibrational band for various crystalline solvates 

(see SI for the assignments of the data to specific solvates). 

Chart 4. Stable Ion Pairs Obtained for LiDFOB from DFT 

Calculations a 

a While C and E look the same superficially, C has the Li + cation in the 

FBO plane and E has the cation in the oxalate plane. 

Table 2. Energies of the Calculated Ion-Pairs (Relative to 

LiDFOB-A) 

MO6−2X ΔE (kJ mol −1 ) B3LYP ΔE (kJ mol −1 ) 

LiDFOB-A 0.0 LiDFOB-A 0.0 

LiDFOB-B 60.4 LiDFOB-B 59.5 

LiDFOB-C 51.0 LiDFOB-C 52.2 

LiDFOB-D 52.2 LiDFOB-D 51.6 

LiDFOB-E 62.6 LiDFOB-E 65.1 

MO6−2X-SMD ΔE (kJ mol −1 ) B3LYP-SMD ΔE (kJ mol −1 ) 

LiDFOB-A 0.0 LiDFOB-A 0.0 

LiDFOB-B 33.5 LiDFOB-B 22.7 

LiDFOB-C 32.5 LiDFOB-C 35.9 

LiDFOB-D 28.4 LiDFOB-D 28.0 

LiDFOB-E 38.1 LiDFOB-E 37.4 

4. CONCLUSIONS 

Numerous crystal structures of LiDFOB solvates are reported 

which provide significant insight into the manner in which the 

DFOB − anion coordinates Li + cations. These solvates have 

been extensively characterized with Raman spectroscopy to link 

the vibrational bands with specific forms of DFOB − ...Li + cation 

coordination. This work has been complemented with DFT 

calculations to identify the origin (vibrational modes) of the 

DFOB − anion vibrational bands. 

5529 

■ 

Article 

ASSOCIATED CONTENT 

*S Supporting Information 

Gaussian file for the DFOB − anion and LiDFOB-A ion pair to 

visualize anion vibrational modes; calculated Raman/IR 

vibrational information for the LiDFOB-A ion pair; differential 

scanning calorimetry (DSC) heating traces for the solvates; 

tables of bond lengths and angles for the CIP and AGG 

solvates; full Raman spectra for the AGG solvates; ion packing 

diagrams, additional details of the structure determination and 

crystallographic data (PDF) and crystallographic information 

files (CIFs) for the crystalline solvate structures (CIFs may be 

readily viewed with freeware programs such as Mercury or 

Jmol). This material is available free of charge via the Internet 

at ■http://pubs.acs.org. 

AUTHOR INFORMATION 

Corresponding Author 

*Tel: 919-513-2917. E-mail: whender@ncsu.edu. 

Notes 

The authors declare no competing financial interests. 

■ ACKNOWLEDGMENTS 

The authors wish to express their gratitude to the U.S. 

Department of Energy (DOE) Batteries for Advanced Transportation 

Technologies (BATT) Program which fully supported 

the experimental portion of this research under Award 

No. DE-AC02-05-CH11231. The Swedish Energy Agency is 

acknowledged for funding the computation portion of this 

research via a VR/STEM grant and SNIC for the allocation of 

computational resources. The authors also wish to thank the 

Department of Chemistry of North Carolina State University 

and the State of North Carolina for funding the purchase of the 

Apex2 diffractometer. 

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